The present invention pertains generally to a system for locating and tracking moving objects. More particularly, the present invention pertains to systems and methods that are useful for locating the position of soldiers or emergency personnel in an urban environment. The present invention is particularly, but not exclusively, useful as a portable, accurate system capable of being rapidly setup to track and locate the position of soldiers or emergency personnel anywhere within an urban environment.
There are many circumstances in which there is a need to establish the accurate positioning and tracking of movable objects or individuals. This is particularly so when the individual or object is moving in a hostile or dangerous environment. One example is when a soldier enters a structure in an urban environment during a military operation. Another example is when a SWAT team member enters a structure during a rescue operation. In situations such as these, there is a need to determine and track the position of an object (i.e. a soldier or SWAT team member) with accuracies of approximately one meter. Although an object""s position can be accurately determined outdoors using the current global positioning system (GPS), the GPS system is unsuitable, without augmentation, for locating moving objects indoors at accuracies of approximately one meter.
A system that can accurately determine an object""s position in an urban environment can be used to provide soldiers/rescue personnel with situational awareness. More specifically, a desirable system would provide each soldier/rescuer with information regarding the position and movements of other soldiers/rescue personnel in the area. Of course, it is important that the soldier""s position is not broadcast to hostile forces. For example, a positioning system that requires a soldier to carry a continuous wave signal transmitter is unsuitable for operations within a hostile urban environment because a hostile party could receive the continuous wave and locate the soldier.
To accurately locate and track objects or individuals inside or adjacent to a structure, the tracking signal that is used by the system must have good penetration and little distortion through the walls and other features of the structures. Lack of adequate signal penetration can result in a loss of signal strength which in turn can cause unacceptable location errors. Also, the signal should have low deflection (refraction and diffraction) to reduce the presence of multipath signals which limit location accuracy. Further, to locate an object""s position accurately indoors, a system must provide sufficient coverage, and be able to acquire the signals quickly.
Unfortunately, radiofrequency (RF) systems using high frequency signals are limited in their ability to penetrate the walls and features of a structure. Also, because high frequency signals have wavelengths that are much shorter than the size of typical structural features such as rooms, hallways and staircases, these features can act as waveguides for the high frequency waves, altering the path of the signal. On the other hand, low frequency RF signals offer the potential to penetrate the walls and features of a structure and overcome inaccuracies due to fading and path length perturbations caused by diffraction and reflection. Further, since the wavelength of the low frequency waves are approximately the same or greater than the size of typical structural features, the features do not act as waveguides for the low frequency waves. Consequently, low frequency RF signals having wavelengths approximating the size of structural features are preferred over high frequency signals for use in and around structures.
Traditional positioning technologies use time-of-arrival and the angle-of-arrival methods. In a typical time-of-arrival system, the system measures the time of arrival of a marker modulated onto a signal to determine range. However, in time-of-arrival systems, increased resolution can only be obtained at the expense of increased bandwidth. By way of example, for a desired locating accuracy of one meter, a typical ranging system based on time of arrival requires a bandwidth on the order of tens of MHz. Unfortunately, this much bandwidth (tens of MHz) is unavailable at the low frequencies required for indoor use.
Another traditional positioning technology is the angle-of-arrival system. Typically, the angle of arrival is measured with array antennas or spinning real-aperture antennas. To achieve an unambiguous angle measurement commensurate with a one meter cross-range resolution at a one kilometer distance, each individual antenna (or array) must be on the order of 15 wavelengths across. Consequently, for the low frequency RF signals required for indoor locating, each antenna would be quite large and costly. Such large antennas are unsuitable when a locator system having small, portable equipment that can be setup quickly is desired.
Considering the above, it is an object of the present invention to provide a wireless system for locating and tracking the position of a soldier within an urban environment with accuracies of approximately one meter. Another object of the present invention is to provide a wireless system for accurately locating the position of a soldier that uses penetrating, low frequency RF signals, and requires only a minimal amount of bandwidth. Still another object of the present invention is to provide a wireless system for accurately locating and tracking the position of a soldier who is situated inside or adjacent to a structure without revealing the soldier""s location to hostile forces. It is still another object of the present invention to provide a wireless locating and tracking system that can be configured to provide situational awareness to personnel situated inside or adjacent to a structure. Yet another object of the present invention is to provide a wireless locating system that is easy to use, relatively simple to implement, and comparatively cost effective.
A system and method for locating and tracking a movable object such as a soldier in a hostile, urban environment includes a receiver positioned on the movable object for movement therewith and a plurality of signal transmitters. For the present invention, the signal transmitters are mutually dispersed at known locations and each signal transmitter is configured to generate a low frequency electromagnetic signal. More specifically, a low frequency signal is used to ensure that the signal will penetrate into buildings in the urban environment. Additionally the frequency of the signal is selected to prevent features of the urban environment from acting as waveguides for the signal and altering the path of the signal. These features of the urban environment that can act as waveguides include rooms, hallways, staircases and passageways between large buildings. For the present invention, the signals generated by the transmitters have a wavelength that is substantially longer than the pertinent dimensions of the urban environment features to prevent these features from acting as waveguides. A typical frequency for use in the present invention is between approximately 500 kHz and approximately 2 Mhz.
Importantly, each signal transmitter is synchronized with the other signal transmitters to allow the generated signals to be in-phase with each other as they are generated by the signal transmitters. On the other hand, when the transmitted signals reach the receiver, the transmitted signals will no longer necessarily be in-phase. This is because each signal will most likely travel a different distance to reach the receiver.
As indicated above, for the present invention, the location of each signal transmitter must be known. For this purpose, each signal transmitter can be bundled with a GPS locating device to self-survey. After acquiring its own location, each signal transmitter relays its location information (along with the low frequency signal) to the receiver. For example, the transmitter location information can be modulated on the low frequency signal. Thus, the system is designed to accommodate an application wherein the transmitters are placed at outdoor locations where accurate GPS location is possible, yet the soldier (and receiver) to be located may be indoors where the accuracy of GPS is inadequate. More specifically, transmitters can be placed on soldiers, vehicles, and equipment and activated (when the transmitters are outdoors) to generate the low frequency signal. In some situations, it is contemplated by the present invention that the transmitters will be xe2x80x98droppedxe2x80x99 by soldiers at outdoor locations within the urban environment. For example, a soldier can drop a transmitter outside a structure as the soldier enters the structure. Thus, with the dropped transmitters positioned away from soldiers, hostile forces are prevented from identifying soldier locations by homing in on the transmitters.
At the receiver, each transmitted signal is received. For each received signal, phase related information and the transmitter location information are extracted from the signal. Also at the receiver, the phase related information and transmitter location information are processed to determine the location of the receiver. In one embodiment of the present invention, the receiver includes an on-board oscillator that is synchronized with the signal transmitters. In this embodiment, the phase of each received signal is measured with respect to the on-board oscillator (i.e. actual phase delays are measured) to determine the distance that each signal has propagated from its respective transmitter to the receiver. It is to be appreciated that each actual phase delay can represent several possible propagation distances, with the possible propagation distances differing by a distance equal to the wavelength of the low frequency signal. As detailed further below, these phase-related ambiguities can be eliminated, allowing the location of the receiver (and soldier) to be calculated from the propagation distances using triangulation algorithms that are well-known in the pertinent art.
In an alternate embodiment of the present invention, an on-board oscillator is not required. Instead, relative phase delays between received signals are used to determine the location of the receiver (and soldier). For a system having three transmitters, it is to be appreciated that three relative phase delays can be calculated. More specifically, a first phase delay for signals from transmitters one and two, a second phase delay for signals from transmitters one and three and a third phase delay for signals from transmitters two and three can be calculated.
Each relative phase delay is indicative of a differential range estimate for the transmitter pair used to calculate the relative phase delay. Specifically, the differential range estimate indicates the difference between the distance from the receiver (and soldier) to one transmitter in the pair and the distance from the receiver to the other transmitter in the pair. It is to be appreciated that phase related ambiguities similar to those described above may also be present in the differential range estimates. As detailed further below, these phase related ambiguities can be resolved and the location of the receiver (and soldier) can be calculated from the differential range estimates using triangulation algorithms that are well-known in the pertinent art.
In each of the embodiments described above, the phase related ambiguities can be eliminated by a processor to find the real receiver position. It is to be appreciated that the number of ambiguities will depend on the transmitter signal wavelength and the coverage area. Several techniques can be used to reduce or eliminate the ambiguities. For example, increasing the number of transmitters will generally reduce the number of ambiguities. A preferred technique involves using an algorithm such as the maximum likelihood method (MLM) to eliminate the phase related ambiguities and find the real receiver position. Another technique involves determining an initial position for the receiver, followed by tracking the movement of the receiver relative to the initial position. This technique allows for some of the ambiguous positions to be eliminated as improbable in light of any known limitations on receiver movement. For example, it can be assumed that the soldier and receiver do not move faster than an average soldier""s foot speed, plus a factor of safety.
Another technique for eliminating ambiguities involves using transmitters that transmit multiple frequencies. Here, each frequency produces a set of possible receiver positions. The set of possible receiver positions produced at one frequency can then be compared to the set of possible receiver positions produced at a second frequency and any possible receiver positions that are not common to both sets can be eliminated as ambiguities. Once the ambiguities have been eliminated, the remaining position is the real position of the receiver relative to the signal transmitters. It is to be appreciated that a combination of the above-described techniques can be used to reduce or eliminate phase related ambiguities.
Once the real position of the receiver is calculated, the real position can be displayed at the receiver for use by the soldier or the receiver position can be transmitted to a central location. Preferably, a low probability of intercept (LPI) waveform is used to transmit the receiver location to the central location to prevent hostile parties from intercepting the location of the soldier. For the purposes of the present disclosure, the term xe2x80x98low probability of intercept waveformxe2x80x99 is defined as a waveform which, because of its low power, high directivity, frequency variability or other design attributes, make it difficult to be detected or identified by passive devices. A burst communication using a wide bandwidth spread spectrum is an example of a suitable LPI waveform for use in the present invention.